Method for protecting metals against corrosion



United States Patent M 3,011,957 LETHGD FOR PROTECTING METALS AGAINST CDRROSION Eugene Eigenmann, 18 Rue Pierre Guerin, Paris, France No Drawing. Filed Nov. 28, 1958, Ser. No. 776,688 Ciairns priority, application France Nov. 29, 1957 4 Claims. .(Cl. 204-40) The present invention relates to an electrolytic method for treating metals in order to protect them against corrosion.

A large number of permanent coatings are known, which can be applied to metallic materials in order to protect them against corrosion in the atmosphere or the like, caused by the presence of various aggressive products. These treatments can be adopted for various purposes, for exmple for decorative coverings, for adhesive carriers for an organic covering or lubricant, for protecting resistant surfaces against wear or for such surfaces which have special electrical or thermal properties, or finally for protecting layers against corrosion.

Among these coverings, phosphates are currently used, which serve mainly as carriers for paints. The relatively porous surface obtained in this manner possesses excellent adhesive power for paints, and if the phosphate protecting layer becomes damaged, corrosion is prevented below the surface sldn. Hitherto these coverings have been obtained by simply bringing the metallic carrier into contact with aqueous solutions which generally contain phosphates, phosphoric acid and various reaction accelerators. The known methods for applying phosphates are dipping, spraying with a spraying gun, as well as brushing on or scouring.

The metallic coverings adopted for protecting against corrosion consist essentially of bringing at least two metals onto each other, the basic metal and at least one protecting metal. In other respects corrosion is in general an electrochemical process, and this allowed a known theory to he established concerning the mechanism of protection against corrosion, based on the metals arranged in series in accordance with their electrochemical potentials. According to this well-known theory, when two metals lie on each other, it is the metal electro-negative with respect to the applied metal which is attacked before the applied metal in the case of corrosion, and this attack proceeds further until the electro-negative metal has completely disappeared, before the attack on the electro-positive metal begins. The terms electro-negative and electropositive are naturally relative terms between the metals in question.

in particular with regard to iron, the metals currently used for its protection are zinc, cadmium and tin. Cadmium is electro-negative with respect to iron. It is therefore the cadmium that will be first attacked in the case of corrosion later. Tin on the other hand is electro-positive, unless organic acids are present, whereby it becomes electro-negative.

Anodic coverings, i.e. electro-nega-tive metals, give selfprotection, i.e. protection against corrosion is ensured even in cases of breakage, cracking, porosity or too little thickness of the protecting metal. The duration of protection is thus limited; it is essentially proportional to the thickness of the covering.

On the other hand, cathodic coverings of electro-positive metals have unlimited duration of protection, as long as the covering is not damaged mechanically. In the case of cracks, porosity or other damage, rapid decomposition occurs in consequence of the attack from below of the basic metal to be protected. On the other hand, the metals described as cathodic are in general not subject to chemical attack in the surroundings usually met with.

The aim of the invention is to provide a method for pro- 7 3,011,9 5? Patented Dec. 5, 1961 tecting metals against corrosion, this method combining the special qualities of each kind of the adore-mentioned coverings. This method thus allows, under all circumstances, of obtaining a considerably greater resistance to corrosion, than is obtained by the known methods. The main feature of the invention consists in an electrolytic treatment, with high current density, of the cleaned metals in an aqueous bath which contains PO -ions, whereby the said metals one after the other form the cathode and then the anode in the electrolysis bath, by the reversing of the electrolysis current. This new, special treatment makes it possible to obtain in a very short time a phosphatizing which has a remarkable resistance against corrosion, as well as forming an excellent adhesive bases for other metallic protecting coverings. According to a preferred form of execution of the method in accordance with the invention, this bath contains also (ZN-dons, the pH- value of this bath being alkaline.

According to the invent-ion, this first dividing layer against corrosion can be improved by the electrolytic precipitation of a first protecting metal, which is preferably elec'tro-negative with respect to the basic metal that has to be protected, whereby this electrolytic precipitation is brought immediately onto the surface previously treated by means of the above-mentioned phosphatizing process. According to a preferred form of execution of the method according to the invention, this protecting metal is cadmium.

A remarkable and hither-t0 unreached resistance against corrosion is obtained because of the second electrolytic precipitation of another protecting metal, which is preferably electro-positive with respect to the basic metal and the metal of the first electrolytic precipitation, whereby this second electro-positive precipitation is produced directly on the previous electro-negative precipitaton. According to a preferred form of execution of the invention, this second protecting metal is tin.

The invention becomes more comprehensible when reading through the following detailed description of one example of execution of the method according to the invention.

In accordance with the method according to the invention, the metals to be treated are first of all thoroughly degreased by some known method, for instance with trichlo-ro ethyl. If the metal parts have been appreciably attacked on their surface, it will be well to pickle them in the manner usual for electrolytic treatment, for example with diluted hydrochloric acid for iron and its alloys, or with diluted sulphuric acid. After rinsing in Water, the characteristic treatment by the method according to the invention can take place. One can for instance adopt a treatment-vat, which contains 25 kg. cyanide of sodium, 22.5 kg. sodium hydroxide, 0.350 kg. phosphate of manganese, l lt. ortho-phosphoric acid and sufiicient water to give 1000 It. solution. This composition is particularly well suited for iron and its alloys. It can naturally be varied within certain limits, depending on the nature of the metal of the pieces to be treated. The utilized electrolysis vat consists of iron and its sides serve as electrodes. The clean metal parts, degreased and piclded, are placed into the bath Without the current being switched on and are left there for a certain time, for instance for five minutes, after which the pieces are connected for two to five minutes to the negative pole of a suitable source of direct current, and then the current is reversed, the pieces being then connected to the positive pole. The treatment is preferably carried out at a temperature of the order of 60 C., with a current density of the order of 10 amps. per square decimetre corresponding to a voltage of the order of 8 to 10 volts. Afterwards the pieces are rinsed in Water. The metal parts treated in this manthan that obtained with the classic phosphatizing. resistance is improved by a precipitation of cadmium by her have already very good resistance to corrosion, better This electrolysis in a classic alkaline cyanide cadmium plating bath, to which silver nitrate is preferably added, approximately 15 grams to l000-litres. This cadmium plating is done under the usual normal conditions. The pieces are rinsed in water, and if it is desired to obtain the maximum resistance against corrosion, they are treated in an alkaline electrolytic tinning bath of the classic type and under the usual conditions. After the rinsing in water, preferably warm water, the pieces are dried as usual, for instance in sawdust.

In this way a composite protecting precipitation obtained with perfect adhesive properties and of the de-- sired thickness, for instance of the order of 20 microns. The difiusion of the tin or of the cadmium in the iron is not perceptible after treatment by the method according to the invention, not even by spectrography. The brittleness of the basic metal is not diminished, which would have been the case if a certain quantity of tin or cadmium had difiused into it. Nevertheless, under the microscope,

.with a magnification of about 1000 times, a zone influenced by the treatment can be determined.

Precipitations of cadmium and tin on each other have already been used to protect metals containing iron against corrosion, since such metals possess particularly interestingproperties. In fact tin alters very little either in the air or in water, and is fairly resistant to the usual external agents. Besides that, it gets an anodic character in-acid surroundings. Cadmium is less resistant than zinc in industrial atmospheres, but it is more resistant in sea air and particularly in a tropical climate. Depending on the special conditions of application, it may in some cases be possible to replace the cadmium precipitation 'by a zinc precipitation. The clearly increased protection obtained when adopting the method according to the invention,

appears therefore to depend actually on the electrolytic phosphatizing treatment. It is in fact known that, in so far as protection against corrosion is concerned, the state of the surface of the basic metal has a considerable influence. The chemophysical cleanliness is of great importance, as is also the degree of polish of the treated surface, the polishing giving a better resistance, except in the case of thick coverings. 1

Thus test plates of iron, with a coating of the thickness of 5 to 12 microns obtained by successive treatment by the method according to the invention, by phosphatizing, cadmium plating and tinning, will prove resistant for 750 hours without showing a trace of corrosion, during a test in salt vapours of 21% .at 35 C. (French standard X41002). Similar plates, treated with the usual electrolytic methods, do not resist longer than 250 to 300 hours.

The composite protecting layer obtained by the method according to the invention, has also remarkable mechanical properties of resistance. Thus cable protecting coverings of 4.8 to,3.2 and 2.4 mm. for airplane construction can be inserted and drawn after the treatment. They withstood the same test with salt vapour for more than 300 hours, whilst the cable protecting coverings treated with the usual electrolytic methods, showed traces of corrosion almost immediately under the same conditions.

It is evident that the invention is by no means restricted to the described example. it can be subjected to numerous variants which are accessible to theexpert, according to the applications coming into question, without departing from the scope of the invention.

Thus, the composition of the phosphatizing bath may be altered'within fairly wide limits, depending on the to be protected in an alkaline, aqueous bath containing 25 kg. of sodium cyanide, 22.5 kg. of sodium hydroxide, 0.35 kg. of manganese phosphate and 1.0 litre of orthophosphoric acid for each 1000 litres of the bath, passing an electric current through the metal to be protected in said bath with a current density of approximately 10 amperes per square decimeter with the metal to be protected forming successively the cathode and then the anode so as to phosphatize the metal to be protected, rinsing-oft the phosphatized metal, electrolytically plating the phosphatized metal with a first protective metal which is electro-negative with respect to said .phosphatized metal, and then electrolytically plating the thus plated metal with a second protective metal which is electro-positive with respect to the metal to be protected and said first protective metal.

2. A method as in claim 1; wherein said first protective metal is cadmium.

3. A method as in claim 1; wherein said second pro tective metal is tin.

4. A method as in claim 1;wherein said first and second protective metals are cadmium and tin, respectively.

References Cited in the file or" this patent UNITED STATES PATENTS OTHER REFERENCES Metal Industry, May 12, 1944, page 299. 

1. A METHOD OF PROTECTING METALS AGAINST CORROSION COMPRISING THE STEPS, IN SEQUENCE, OF IMMERSING THE METAL TO BE PROTECTED IN AN ALKALINE, AQUEOUS BATH CONTAINING 25 KG. OF SODIUM CYANIDE, 22.5 KG. OF SODIUM HYDROXIDE, 0.35 KG. OF MANGANESE PHOSPHATE AND 1.0 LITRE OF ORTHOPHOSPHORIC ACID FOR EACH 1000 LITRES OF THE BATH, PASSING AN ELECTRIC CURRENT THROUGH THE METAL TO BE PROTECTED IN SAID BATH WITH A CURRENT DENSITY OF APPROXIMATELY 10 AMPERES PER SQUARE DECIMETER WITH THE METAL TO BE PROTECTED FORMING SUCCESSIVELY THE CATHODE AND THEN THE ANODE SO AS TO PHOSPHATIZE THE METAL TO BE PROTECTED, RISING-OFF THE PHOSPHATIZED METAL, ELECTROLYTICALLY PLATING THE PHOSPHATIZED METAL WITH A FIRST PROTECTIVE METAL WHICH IS ELECTRO-NEGATIVE WITH RESPECT TO SAID PHOSPHATIZED METAL, AND THEN ELECTROLYTICALLY PLATING THE THUS PLATED METAL WITH A SECOND PROTECTIVE METAL WHICH IS ELECTRO-POSITIVE WITH RESPECT TO THE METAL TO BE PROTECTED AND SAID PROTECTIVE METAL. 